PROJECT SUMMARY Chemokines control the migration and localization of leukocytes and play fundamental roles in regulating immune and inflammatory responses. One such chemokine, CXCL12, promotes multiple steps in the growth of many primary tumors and progression to metastasis by binding to C-X-C chemokine receptor type 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3), which are upregulated in many cancers. Unlike most G protein- coupled receptors (GPCRs), CXCL12-bound ACKR3 signals only via arrestins. In response to CXCL12, ACKR3 is phosphorylated by intracellular GPCR kinases (GRKs) which subsequently recruit arrestins. The arrestins serve as scaffolds that promotes growth pathways critical for cell survival, proliferation, and migration. Arrestins also drive receptor internalization, during which CXCL12 is trafficked to lysosomes and degraded. Afterwards, the empty receptor is recycled to the cell surface where it maintains a relatively stable concentration. This process results in the scavenging or uptake of CXCL12 from the extracellular space and is important for maintaining the responsiveness of CXCR4-expressing cells in the context of normal physiology as well as tumor metastasis. In this proposal, the Tesmer and Handel labs, with deep expertise in GRKs and chemokine receptor structure and function, respectively, join forces to better understand the molecular mechanisms underlying ACKR3 phosphorylation by different GRKs, how arrestins interact with the resulting phosphorylation ?barcodes? installed in the C-terminus of the receptor, and what the cellular consequences are. They have discovered that GRK2 and GRK5 phosphorylate activated ACKR3 at distinct regions of its cytoplasmic tail. Moreover, phosphorylation enhances binding to both arrestin2 and 3. Arrestin2 recruitment has not been reported before, and thus its functional significance remains to be elucidated. They have further isolated complexes of ACKR3 with both arrestin as well as with GRK2?G???that are of suitable quality for high resolution cryo-electron microscopy (cryo-EM) reconstructions and have shown that G?? subunits alone can form a strong interaction with ACKR3 of unknown function. In Aim1, cryo-EM structures of CXCL12-activated ACKR3 will be determined in complex with various GRKs, with focus on GRK2, and with G??. In Aim2, cryo- EM will be used to examine arrestin complexes with phosphorylated ACKR3. In Aim 3, hypothesis driven cell- based assays of ACKR3 function and unbiased mass spectrometry approaches will be used to systematically investigate mechanisms by which these proteins control arrestin-mediated signaling and scavenging by ACKR3 and determine if there is specific GRK and arrestin isoform control of ACKR3 function. The successful conclusion of this proposal will result in the first structure of an atypical chemokine receptor in complex with its intracellular signaling partners as well as unprecedented insights into the molecular mechanisms of a therapeutically important receptor that may ultimately aid in the development of new cancer treatments.